The perceptron learning algorithm is an example of supervised learning. This kind of approach does not seem very plausible from the biologist´s point of view, since a teacher is needed to accept or reject the output and adjust the network weights if necessary. Some researchers have proposed alternative learning methods in which the network parameters are determined as a result of a self-organizing process. In unsupervised learning corrections to the network weights are not performed by an external agent, because in many cases we do not even know what solution we should expect from the network. The network itself decides what output is best for a given input and reorganizes accordingly.